What is the recommended angle to set the Pole angle to when setting up an IK rig?
In the ideal case, you should have the pole value set to zero, reaching the desired behaviour by tweaking the bones roll in edit mode, and it's a habit to choose positive X axis as main bone rotation. The more your rotations happen on a single axis, the easier is to edit the animation with the graph editor, and this is the reason why - 90 and 90 degrees are considered "good values" (they "swap" the main rotation from X to Z axis). In a Pro rig the pole set to zero lets the animator to tweak it for special purposes and reset the default value easily. Aside from theese considerations, if the rig is used by its creator, you can set that value as you want, without any other bad repercussion (as far as I know).